Continuity mistake: In the scene where Sir Galahad is about to enter Castle Anthrax, it's pouring like there's no tomorrow. Although when he falls against the door and is about to open there is no rain, and no Niagara Falls pouring down the door. As a matter of fact, it's pretty dry on the door and almost a completely clear sky.
Continuity mistake: After Sir Lancelot has gone on a rampage at the wedding party, the bride is first seen with a clean face. Then in the close-up she has loads of bright red Pythonesque blood coming from her mouth and running onto her chin. In the next shot, the blood has disappeared again.
Add timerabid anarchist
Revealing mistake: In the scene with the French taunting they attack the castle. Shortly afterwards they run away and hide on the hill slope. When Arthur is trying to lie down he nearly guts one of the knights with his sword and just after that when he gets his sword in front of him you can see that it is a fake wobbly sword.
Continuity mistake: When the rabbit kills the first knight that is sent to kill it, the rabbit gets blood all over its mouth. However, when the knights charge the rabbit, in the shot of the rabbit watching them, the blood has disappeared. It reappears in the shots where it is slaughtering the other knights.
Revealing mistake: After King Arthur cuts off both the Black Knight's arms and both his legs, as Arthur rides away and the Knight is struggling on the ground, as he turns sideways you can see both his hands, held behind his back, and the depression in the ground where he is standing in a hole. This is how the effect was achieved.
Add timeThe Doctor
Revealing mistake: When the Black Knight kills the other knight by throwing a sword through his head, you can see that the blood is coming from the sword, not the wound itself. Likely the actor had a blood tube in his hand, and let the blood run down the sword and splatter onto his face.
Other mistake: It's revealed that the Knights who say Ni can't say "it" (at least, after they become the Knights Who Say... that other thing), but they still say "it," most notably when they're telling Arthur to place the second shrubbery next to the first one - "When you have found another shrubbery, you must place IT here, beside this shrubbery..."